Abducted children in Iraq are reportedly being sold at markets as sex slaves and being either killed, crucified or buried alive by the Islamic State militants, a United Nations watchdog said on Wednesday.

The terrorist organization is horrifically using teenage boys as suicide bombers, bomb makers, informants or human shields against U.S.-led air strikes, the U.N. Committee on the Rights of the Child said.

"The scope of the problem is huge. We are really deeply concerned at torture and murder of those children, especially those belonging to minorities, but not only from minorities," committee expert Renate Winter told a news briefing, adding that children from the Yazidi sect or Christian communities have been victims as well as Shi'ites and Sunnis.

Amidst all this atrocity, ISIS is also using mentally challenged boys as suicide bombers, Breitbart reported.

"We have had reports of children, especially children who are mentally challenged, who have been used as suicide bombers, most probably without them even understanding," Winter told Reuters. "There was a video placed (online) that showed children at a very young age, approximately eight years of age and younger, to be trained already to become child soldiers."

"Children of minorities have been captured in many places... sold in the market place with tags, price tags on them, they have been sold as slaves," Winter continued. "A large number of children have been killed or badly wounded during air strikes or shelling by Iraqi security forces, while others had died of dehydration, starvation and heat."

In November, an anti-ISIS campaign group had revealed that ISIS was running "terror schools" in Syria, where children as young as five were being trained to become soldiers and suicide bombers.

Training camps at al-Sharea school in Raqqa, the de facto Islamic State capital, have attracted the attendance of more than 300 children, who have either been kidnapped or willingly sent by parents to learn the teachings of the terrorist organization.

In the schools, children are forced to repeatedly chant that Western infidels are meant to be killed and are questioned on whether they would like to be a jihadi fighter or a suicide bomber.

"The recruitment camps have been established by ISIS as an attempt to brainwash the minors and promote extremism among the young generations in order to create an army of loyal followers," a spokesman for Raqqa Is Being Silently Slaughtered told the Sun.

"ISIS has recently stepped up its youth recruitment, a sort of boot camp for young boys in which they are taught combat skills."

"The group persuades parents to send their kids in exchange for sums of money, taking advantage of their need, in the midst of dire living conditions and widespread poverty," he said. "However, not all youth recruitment carried out by ISIS is voluntary. In some cases young boys are taken to boot camps without parental consent."

After being recruited at mosques and given a 45-day course of intense training at various camps, the boys are sent to learn combat training for a month, according to sources.

While most head straight to the front lines after that, others are allotted to guard checkpoints. "Many of these boys are time bombs waiting to go off," the spokesman added.